The present invention relates to an electro-mechanical thread or yarn supervisory apparatus to check for the presence or absence of thread being supplied to a textile machine, for example to a spooling machine or the like, and having a mechanical element which is in physical contact with the thread to be supervised.
Thread supervisory apparatus are used in many branches of textile manufacture. Such apparatus check for the presence of a thread and provide a signal if the thread should fail to be detected, so that the production machine to which the supervisory apparatus is coupled may be stopped. Such apparatus is also at times referred to as a "stop motion" device. Three main groups or categories of thread supervisory apparatus are in common use: mechanical apparatus, electro-mechanical apparatus, and purely electrical apparatus.
The mechanical thread supervisory apparatus use a lever system in which the thread or yarn directly controls machine function. Such a system requires precise bearing of the mass to be moved. The mass to be moved, itself, is comparatively great, which results in apparatus which is generally slow and unreliable.
Electro-mechanical thread supervisory apparatus also usually use a lever system which can be constructed to be much lighter, however, than the lever system of purely mechanical apparatus. The lever system only operates an electrical contact. A magnet which forms part of the machine is energized, directly or by means of a relay, the magnet then operating a magnetic clutch, an electric brake, or the like to effect stop motion of the machine.
Purely electrical and electronic stop motion devices do not require movable parts. The thread is sensed without contact therewith, either photoelectrically or capacitatively. So-called operating thread sensors are effective only when the thread is in motion. Small variations in uniformity of the spun thread are used to obtain the signal, so that, during running of the thread, an aperiodic alternating signal is obtained. Such signals are usually not available when the thread is a monofilamentary synthetic thread made with high precision, since such threads are highly uniform. The utility of electronic thread sensors is thus limited and, additionally, such sensors are considerably more expensive than other types.
Mechanical thread sensors have a wide field of utility when many runs of threads are to be supervised, for example in machinery which uses 1,000 to 2,000 threads, for example warp threads of looms.
Mechanical thread supervisory apparatus operate by having a thread engage a rotatably journalled sensing lever by being looped thereabout over a certain looping angle, pressing the lever in a predetermined position. If the thread breaks, a weight or a spring, providing a reset force, becomes effective, returning the sensing lever into a rest position, thus triggering a control signal for the machine. To change the position of the sensing lever into the supervisory position, it is therefore necessary to overcome not only a certain distance, but also a certain reset force. The force with which the thread presses on the lever must be at least slightly above this reset force. This force must be applied by the lever corresponding to a sine function of the looping angle, and must be available by virtue of thread tension in the machine; the maximum force is determined by the maximum permissible thread tension in the running direction of the thread.
Mechanical or electro-mechanical supervisory apparatus in current use require an application pressure on the sensing lever of at least 2 grams. If the reset force is substantially decreased, then small extraneous influences or operating parameters, such as dirt, friction or the like may interfere with proper operation. The reset force, and thus the stop motion action of the sensor also become slow, which is not acceptable in high thread supply speeds.
Thin synthetic threads, typically monofilamentary threads, are currently used in textile machinery with thread tension of only 2 to 3 grams. A thread sensor having only 2 grams pressure would then require the thread to be carried thereabout with a looping angle of 90.degree.. This is frequently impossible from a machinery design point of view and, additionally, may lead to damage of the thread, due to roughing or stretching of the thread. Acceptable looping angles are between 10.degree. to 15.degree.. As a result, however, only one fifth of the thread tension is available on the average to operate the thread supervisory sensor. A thread sensor for thin synthetic threads should, therefore, operate with an application pressure of less than 0.5 gram while still operating reliably and quickly.
All mechanical thread sensors have an additional disadvantage and that is the lack of self-supervision. The thread sensor may fail to stop the machine if the sensing lever is impeded in its freedom of movement, for example by dirt, bending, fluff, slubs, remnants of thread, or the like. Frequently this condition is not discovered until hours later, and then only by chance; in the meanwhile, imperfect textile material has been produced, so that high losses in supplies, materials and production may result. The thread sensor should, therefore, operate in such a manner that is continuously supplies an output signal, even when the thread is proper, and also when its sensing element is impeded in free movement, thus stopping the machine, so that defective thread supervisory apparatus can be replaced in time.
All presently available thread sensors only respond when the thread is entirely missing, or when the thread tension is below the reset force. If, however, the thread should not feel properly, for example due to a defective thread brake, poor spooling, formation of slubs, kinks, or the like, then the thread tension will rise substantially beyond its proper value. This condition is also a substantial defect, which should be signalled, since it may lead to stretching of the thread and hence poor quality of the resulting product. It is thus important in many applications that the thread sensor respond not only when the thread tension disappears, for example due to breakage, but also when a tolerated design upper limit of the thread tension is exceeded.
It is an object of the present invention to provide an electro-mechanical thread sensor which has sufficient reset force in spite of extremely light thread application pressure, which is self-supervisory and which responds not only when there is a breakage in thread, but also when the thread tension becomes excessive.